1. Field of the Invention
This invention is concerned with control of industrial processes, and in particular with controlling soldering processes used to interconnect components in electronic circuits.
2. Description of the Related Art
Modern microelectronic circuits generally comprise electronic components and devices interconnected via a printed wiring board (PWB), which includes copper lines, that serve as interconnection wires, and surface contact pads to which component and device leads are soldered. The PWB may include metallic surface pads that serve other functions, edge connections or circuit test points, for example. The PWB lines and pads are formed on a dielectric substrate, typically a polymer laminant or a ceramic, by a combination of electroless copper deposition, photolithography and copper electrodeposition. Multiple layers of circuitry are often laminated together and interconnected by via holes, which typically have copper plated walls and are filled with solder. A surface finish of an oxidation-resistant metal is generally applied to the PWB copper pads to inhibit formation of a surface oxide that would reduce solderability. Typical surface finishes include electroless nickel/gold, immersion tin, and electroplated tin-bismuth.
Component and device leads are typically soldered to PWB contact pads by a reflow soldering process that involves applying a predetermined amount of solder paste to the PWB contact pads, populating the PWB by placing the components and devices such that their input/output (I/O) leads are aligned with the appropriate PWB contact pads, and heating the populated PWB assembly to reflow (melt) the solder in the solder paste. The predetermined amount of solder paste is typically applied to PWB contact pads via an automated syringe dispenser, or via a stencil and squeegee. The predetermined amount of solder paste may vary depending on the specific components and devices to be soldered. A typical device is a ball grid array (BGA) having an array of solder balls attached to contact pads on one side of the device. Many BGA devices have a high I/O lead count so that the distance between adjacent contact pads (pitch) is very small.
Solder pastes generally contain a powder of small solder spheroids that coalesce during reflow to form the solder mass that accounts for a large fraction of the solder in the solder joint. The solder paste also contains a soldering flux that dissolves oxides from the metallic surfaces involved in the soldering process. Soldering fluxes typically contain an organic halide that is activated at a sufficiently elevated temperature to yield an organic acid and a free halogen species, which are effective for dissolving metallic oxides. Solder pastes may also contain ingredients to provide desirable rheological properties, and to inhibit reoxidation of the metallic surfaces as the flux is consumed during solder reflow. In particular, the solder paste must be sufficiently stiff (and tacky) to hold components and devices in place prior to solder reflow, and to resist slumping that can lead to electrical shorts due to solder bridging between adjacent contact pads.
Reflow soldering is generally performed in a reflow oven that includes a metallic belt conveyor for transporting the electronic assembly through the oven, and has different heating zones to enable the assembly to be heated according to a predetermined temperature-time profile. In some cases, the reflow oven or a section thereof may be blanketed with nitrogen to inhibit oxidation of the surfaces to be soldered during the reflow process.
Reflow soldering is a complicated process requiring sufficient flux activity just prior to the time that the solder reflows (melts) to dissolve surface oxides (on PWB pads, component leads and solder particles in solder paste) so that strong solder joints having low electrical resistance are attained. The flux activity required depends strongly on the amount and type of oxides on the surfaces to be joined. For a reliable reflow soldering process, it is necessary to utilize the appropriate solder paste and reflow conditions for the particular assemblies to be soldered.
Reflow soldering problems associated with flux activity generally fall in one of four categories: (1) flux in the solder paste loses activity via chemical reactions during storage so that the remaining activity at the time of use in the soldering process is inadequate; (2) flux in the solder paste activates too early (at a temperature below the solder reflow temperature) so that the surfaces to be joined re-oxidize prior to solder reflow; and (3) the flux does not activate sufficiently at the reflow temperature; and (4) flux remains active after the soldering process, causing corrosion and/or electromigration that can lead to failure of the circuit due to electrical shorts, opens, or excessive interconnection resistance.
Another soldering process currently used by industry to assemble electronic devices is wave soldering, in which a PWB populated with devices (surface mount and/or through-hole) is passed over a wave of molten solder. Wave soldering is prone to the same types of flux activity problems as reflow soldering.
Available methods for controlling soldering processes include: (1) ball/lead shear or pull tests involving measurements of the force required to produce solder joint failure; (2) solderability determination via solder spread tests, wetting balance tests or sequential electrochemical reduction analysis (SERA); (3) detection of solder flux residues via surface insulation resistance (SIR) measurements, halide analysis, ionic cleanliness testing, electrochemical migration tests, and copper mirror and copper plate corrosion tests; (4) measurements of solder paste physical and chemical properties, including viscosity, specific gravity, percent halogen content, tackiness, acid number, pH and impedance spectroscopy (to detect solder ball oxidation); and (5) flux chemistry analysis via chromatography (GC, HPLC, IC and GPC), light spectroscopy (UV, FTIR, Raman and AA), thermographic analysis (DSC, TGA), and wet chemical analysis.
All of these methods are applied before or after the soldering process and provide information about only one parameter relating to soldering process performance. None of these methods provide information about the soldering process itself. This is equally true for the wetting balance test in which an attempt is made to simulate the soldering process by measuring the wetting force (via the weight of the solder meniscus) when a test specimen is brought into contact with molten solder. The wetting balance provides only an indication of solderability, is difficult to apply to PWB's and BGA's, and does not adequately simulate an actual soldering process in terms of specimen geometry, flux application, pre-heating (temperature profile), and specimen thermal inertia. There is clearly a need for a method of measuring the performance of soldering processes so that they can be optimized with respect to all of the important variables.